1. Field of the Invention
The present invention relates to a vehicle air conditioning apparatus which performs air conditioning in a vehicle passenger compartment so that a window glass of the vehicle is not clouded by controlling a heating means and a cooling means.
2. Description of the Related Art
A vehicle air conditioning apparatus adjusts the temperature and the humidity in the vehicle passenger compartment by a vaporizer which cools the air and performs dehumidification by utilizing the heat of vaporization of cooling medium, together with a heater which heats the air by utilizing the heat of engine cooling water or the like.
When the temperature is low and the vaporizer is controlled so that the cooling performance of the vaporizer is lowered, then dehumidifying performance is reduced as well. Therefore, for example, when the temperature is low and the humidity is high, water vapor in the vehicle passenger compartment may be subjected to dew formation on the window glass.
In order to solve this problem, the vehicle air conditioning apparatus is controlled so that vehicle interior absolute humidity is lower than window glass dew point absolute humidity. For this purpose, it is necessary that the vehicle interior surface temperature of the window glass is determined correctly, and the window glass dew point absolute humidity is calculated from the temperature.
As for the vehicle interior surface temperature of the window glass, the temperature can be measured highly accurately, for example, by directly attaching a temperature sensor to the window glass. However, it is necessary to provide an exclusively equipped temperature sensor as well as a processing circuit and cable wiring for processing a signal from the temperature sensor. Therefore, the system may be expensive.
In view of the above, Japanese Patent No. 3298151 has proposed a method in which the vehicle interior surface temperature of the window glass is estimated by using measured values obtained by an outside air temperature sensor, a vehicle interior temperature sensor, and a solar radiation amount sensor which are essential constitutive elements to control the vehicle air conditioning apparatus. In this method, it is unnecessary to provide any exclusively equipped temperature sensor for detecting the vehicle interior surface temperature of the window glass. Further, it is unnecessary to provide the processing circuit and the cable wiring. Therefore, for example, it is unnecessary to change the hardware specification. This system is extremely advantageous in view of cost.
In the case of the method described above, for example, when environmental conditions such as outside air temperature and solar radiation amount are not suddenly changed, the vehicle interior surface temperature of the window glass is stable. Therefore, the temperature can be estimated highly accurately by using the outside air temperature sensor, the vehicle interior temperature sensor, and the solar radiation amount sensor.
However, when the environmental conditions are suddenly changed, the measured values obtained by the respective sensors are quickly changed, following the environmental change. Then, the vehicle interior surface temperature is promptly calculated from the measured values. However, it takes a certain period of time the actual vehicle interior surface temperature of the window glass to reach a stable temperature due to the influence of the thermal capacity of the window glass or the like. Therefore, the estimated temperature is considerably deviated from the actual vehicle interior surface temperature. Further, the vehicle interior surface temperature of the window glass does not change immediately after the change of the vehicle exterior surface temperature. The temperature changes after the elapse of the response delay time depending on the thickness of the window glass and the coefficient of thermal conductivity of the glass. Therefore, in this case, according to the method disclosed in Japanese Patent No. 3298151, it is impossible to appropriately control the vehicle interior absolute humidity, and the window glass may be clouded.